A conventional magnetic recording medium used in a hard disc drive (HDD) of a computer comprises an underlayer 2, an intermediate layer 3, a magnetic layer 4, and a protective layer 5, sequentially formed on a substrate 1 of aluminum or glass, as schematically illustrated in FIG. 7. Information is recorded in the magnetic layer 4 as a magnetized recording bit on the magnetic recording medium. Of such recording media, a medium in which a direction of magnetization of the recording bit is in a horizontal direction (or in-plane direction) in relation to the substrate surface is called a horizontal (or longitudinal or in-plane) recording medium. A medium in which the magnetization direction of the recording bit is in a vertical direction is called a vertical (or perpendicular) recording medium. All media in practical use to date are of horizontal recording system. However, the vertical recording media are being actively pursued in recent years.
The magnetic layer 4 is generally composed of a polycrystalline thin film of a cobalt system in which fine crystallites of cobalt are formed into a film shape. Each cobalt crystallite has a hexagonal closest packed (hcp) structure of a hexagonal crystal system. To record information by magnetizing the magnetic layer 4 with this structure in the horizontal (in-plane) direction in relation to the surface of the substrate 1, the polycrystalline magnetic film of the cobalt system must be formed so that the axis of easy magnetization, namely the c-axis of the a- and c-axes shown in FIG. 8, orients with preference in the horizontal (in-plane) direction in relation to the substrate 1. In a perpendicular recording medium, the cobalt c-axis must be predominantly oriented vertically to the substrate 1.
It is desirable to minimize the size of each crystal grain to achieve a high recording density. In addition, it is desirable to isolate and separate the crystal grains from each other so that magnetic interaction between the crystal grains decreases. To this end, polycrystalline thin film of a CoCr system has been used for the magnetic layer 4. This cobalt alloy containing 10 to 20% of chromium separates into a cobalt-rich ferromagnetic phase and a chromium-rich nonmagnetic phase by a phase separation. The chromium-rich nonmagnetic phase tends to precipitate at the grain boundary. Thus, the cobalt-rich ferromagnetic crystal grains are made minute, and isolated and separated from each other. To minimize and separate the crystal grains and, at the same time, to accomplish the predominant c-axis alignment in the horizontal (in-plane) direction, an underlayer of a polycrystalline thin film made of chromium or chromium alloy is formed on the substrate of aluminum or glass, and a magnetic layer of CoCr system is formed on this underlayer.
There are two reasons why chromium or chromium alloy has been used for the underlayer. First, the magnetic layer 4 is a material of a CoCr system; the chromium-rich nonmagnetic phase precipitates surrounding the cobalt-rich ferromagnetic crystalline grains. Consequently, chromium of the underlayer 2 aids to minimize and separate the crystalline grains in the magnetic layer 4 by promoting precipitation of the chromium-rich nonmagnetic phase in the magnetic layer 4. Second, the chromium in the underlayer has an effect to predominantly align the c-axis of the cobalt crystal of the magnetic layer 4 in a horizontal (or in-plane) direction. This effect arises from good lattice matching of the chromium with the CoCr alloy crystal although chromium has a body centered cubic (bcc) structure belonging to a cubic crystal system. Such a magnetic recording medium is produced by sequentially forming a chromium underlayer, a CoCr alloy magnetic layer, and a carbon protective layer on a disk shape aluminum substrate with a NiP plating by means of a sputtering method. A magnetic recording medium using a glass substrate is similarly produced by sequentially forming a NiAl seed layer, a chromium underlayer, a CoCr magnetic layer, and a protective layer on a glass substrate by means of sputtering.
In a conventional magnetic recording medium using a CoCr magnetic layer, a chromium underlayer, and a substrate of aluminum or glass, however, the substrate needs to be heated to a temperature between about 200° C. and 300° C., while the magnetic layer is being formed by a sputtering method. Without the substrate heating, a high coercive force Hc required for achieving high recording density cannot be attained. The magnetic layer, however, can be formed with high coercive force by using a relatively high concentration of platinum in the range from 5 to 10% with respect to cobalt atoms contained in the CoCr magnetic layer. Even if the magnetic layer containing relatively high concentration of platinum is used, however, high coercive force cannot be attained without substrate heating when a chromium underlayer is used. It has been theorized that if substrate heating is not executed, phase separation of the CoCr alloy into a cobalt-rich phase and a chromium-rich phase does not sufficiently progress, thus insufficiently isolating the cobalt crystalline grains; predominant in-plane alignment of the c-axis of the cobalt crystals will also be insufficient. Because a plastic substrate cannot allow substrate heating in a deposition process, a magnetic recording medium that copes with high recording density cannot be manufactured using a plastic substrate. A magnetic recording medium using an aluminum substrate or a glass substrate is also more expensive because it requires substrate heating during sputtering. Accordingly, there is a need to provide a way of avoiding substrate heating during the manufacture of a magnetic recording medium having excellent magnetic property. The present invention addresses this need.